Title:The carbon monoxide-rich interstellar comet 2I/Borisov

Abstract: Interstellar comets offer direct samples of volatiles from distant
protoplanetary disks. 2I/Borisov is the first notably active interstellar comet
discovered in our solar system[1]. Comets are condensed samples of the gas,
ice, and dust that were in a star's protoplanetary disk during the formation of
its planets and inform our understanding on how chemical compositions and
abundances vary with distance from the central star. Their orbital migration
moves volatiles[2], organic material, and prebiotic chemicals in their host
system[3]. In our solar system, hundreds of comets have been observed remotely,
and a few have been studied up close by space missions[4]. However, knowledge
of extrasolar comets has been limited to what could be gleaned from distant,
unresolved observations of cometary regions around other stars, with only one
detection of carbon monoxide[5]. Here we report that the coma of 2I/Borisov
contains significantly more CO than H2O gas, with abundances of at least 173%,
more than three times higher than previously measured for any comet in the
inner (<2.5 au) solar system[4]. Our ultraviolet observations of 2I/Borisov
provide the first glimpse into the ice content and chemical composition of the
protoplanetary disk of another star that is substantially different from our
own.